Method of manufacture of semiconductor device and conductive compositions used therein

ABSTRACT

The present invention is directed to a thick film conductive composition comprising: (a) electrically conductive silver powder; (b) Zn-containing additive wherein the particle size of said zinc-containing additive is in the range of 7 nanometers to less than 100 nanometers; (c) glass frit wherein said glass frit has a softening point in the range of 300 to 600° C.; dispersed in (d) organic medium. The present invention is further directed to a semiconductor device and a method of manufacturing a semiconductor device from a structural element composed of a semiconductor having a p-n junction and an insulating film formed on a main surface of the semiconductor comprising the steps of (a) applying onto said insulating film the thick film composition as describe above; and (b) firing said semiconductor, insulating film and thick film composition to form an electrode.

FIELD OF THE INVENTION

This invention is directed primarily to a silicon semiconductor device.In particular, it is directed to a conductive silver paste for use inthe front side of a solar cell device.

TECHNICAL BACKGROUND OF THE INVENTION

The present invention can be applied to a broad range of semiconductordevices, although it is especially effective in light-receiving elementssuch as photodiodes and solar cells. The background of the invention isdescribed below with reference to solar cells as a specific example ofthe prior art.

A conventional solar cell structure with a p-type base has a negativeelectrode that is typically on the front-side or sun side of the celland a positive electrode on the backside. It is well-known thatradiation of an appropriate wavelength falling on a p-n junction of asemiconductor body serves as a source of external energy to generatehole-electron pairs in that body. Because of the potential differencewhich exists at a p-n junction, holes and electrons move across thejunction in opposite directions and thereby give rise to flow of anelectric current that is capable of delivering power to an externalcircuit. Most solar cells are in the form of a silicon wafer that hasbeen metallized, i.e., provided with metal contacts that areelectrically conductive.

Most electric power-generating solar cells currently used on earth aresilicon solar cells. Process flow in mass production is generally aimedat achieving maximum simplification and minimizing manufacturing costs.Electrodes in particular are made by using a method such as screenprinting to form a metal paste. An example of this method of productionis described below in conjunction with FIG. 1. FIG. 1 shows a p-typesilicon substrate, 10.

In FIG. 1(b), an n-type diffusion layer, 20, of the reverse conductivitytype is formed by the thermal diffusion of phosphorus (P) or the like.Phosphorus oxychloride (POCl₃) is commonly used as the phosphorusdiffusion source. In the absence of any particular modification, thediffusion layer, 20, is formed over the entire surface of the siliconsubstrate, 10. This diffusion layer has a sheet resistivity on the orderof several tens of ohms per square (Ω/□), and a thickness of about 0.3to 0.5 μm.

After protecting one surface of this diffusion layer with a resist orthe like, as shown in FIG. 1(c), the diffusion layer, 20, is removedfrom most surfaces by etching so that it remains only on one mainsurface. The resist is then removed using an organic solvent or thelike.

Next, a silicon nitride film, 30, is formed as an anti-reflectioncoating on the n-type diffusion layer, 20, to a thickness of about 700to 900 Å in the manner shown in FIG. 1(d) by a process such as thermalchemical vapor deposition (CVD).

As shown in FIG. 1(e), a silver paste, 500, for the front electrode isscreen printed then dried over the silicon nitride film, 30. Inaddition, a backside silver or silver/aluminum paste, 70, and analuminum paste, 60, are then screen printed and successively dried onthe backside of the substrate. Firing is then carried out in an infraredfurnace at a temperature range of approximately 700 to 975° C. for aperiod of from several minutes to several tens of minutes.

Consequently, as shown in FIG. 1(f), aluminum diffuses from the aluminumpaste into the silicon substrate, 10, as a dopant during firing, forminga p+ layer, 40, containing a high concentration of aluminum dopant. Thislayer is generally called the back surface field (BSF) layer, and helpsto improve the energy conversion efficiency of the solar cell.

The aluminum paste is transformed by firing from a dried state, 60, toan aluminum back electrode, 61. The backside silver or silver/aluminumpaste, 70, is fired at the same time, becoming a silver orsilver/aluminum back electrode, 71. During firing, the boundary betweenthe backside aluminum and the backside silver or silver/aluminum assumesan alloy state, and is connected electrically as well. The aluminumelectrode accounts for most areas of the back electrode, owing in partto the need to form a p+ layer, 40. Because soldering to an aluminumelectrode is impossible, a silver back electrode is formed over portionsof the backside as an electrode for interconnecting solar cells by meansof copper ribbon or the like. In addition, the front electrode-formingsilver paste, 500, sinters and penetrates through the silicon nitridefilm, 30, during firing, and is thereby able to electrically contact then-type layer, 20. This type of process is generally called “firethrough.” This fired through state is apparent in layer 501 of FIG.1(f).

JP-A 2001-313400 to Shuichi et al., teaches a solar cell which isobtained by forming, on one main surface of a semiconductor substrate,regions that exhibit the other type of conductivity and forming anantireflection coating on this main surface of the semiconductorsubstrate. The resulting solar cell has an electrode material coatedover the antireflection coating and fired. The electrode materialincludes, for example, lead, boron and silicon, and additionallycontains, in a glass frit having a softening point of about 300 to 600°C., and one or more powders from among titanium, bismuth, cobalt, zinc,zirconium, iron, and chromium. These powders have an average particlesize range from 0.1 to 5 μm. Shuichi et al. teaches that powders havingan average particle size of less than 0.1 μm will have poordispersibility inside the electrode, and the electrode will exhibitinadequate adhesive strength (tensile strength).

Although various methods and compositions for forming solar cells exist,there is an on-going effort to increase electrical performance and atthe same time improve solder adhesion in solar cell applications. Thepresent inventors desired to create a novel composition(s) and method ofmanufacturing a semiconductor device which improved both electricalperformance and solder adhesion.

SUMMARY OF THE INVENTION

The present invention is directed to a thick film conductive compositioncomprising: (a) electrically conductive silver powder; (b) Zn-containingadditive wherein the particle size of said zinc-containing additive isin the range of 7 nanometers to less than 100 nanometers; (c) glass fritwherein said glass frit has a softening point in the range of 300 to600° C.; dispersed in (d) organic medium.

The present invention is further directed to a semiconductor device anda method of manufacturing a semiconductor device from a structuralelement composed of a semiconductor having a p-n junction and aninsulating film formed on a main surface of the semiconductor comprisingthe steps of (a) applying onto said insulating film the thick filmcomposition as describe above; and (b) firing said semiconductor,insulating film and thick film composition to form an electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram illustrating the fabrication of asemiconductor device.

Reference numerals shown in FIG. 1 are explained below.

-   -   10: p-type silicon substrate    -   20: n-type diffusion layer    -   30: silicon nitride film, titanium oxide film, or silicon oxide        film    -   40: p+ layer (back surface field, BSF)    -   60: aluminum paste formed on backside    -   61: aluminum back electrode (obtained by firing backside        aluminum paste)    -   70: silver or silver/aluminum paste formed on backside    -   71: silver or silver/aluminum back electrode (obtained by firing        back side silver paste)    -   500: silver paste formed on front side    -   501: silver front electrode (formed by firing front side silver        paste)

DETAILED DESCRIPTION OF THE INVENTION

The main components of the thick film conductor composition(s) areelectrically functional silver powders, zinc-containing additive, andglass frit dispersed in an organic medium. Additional additives mayinclude metals, metal oxides or any compounds that can generate thesemetal oxides during firing. The components are discussed herein below.

I. Inorganic Components

The inorganic components of the present invention comprise (1)electrically functional silver powders; (2) Zn-containing additive(s);(3) glass frit; and optionally (4) additional metal/metal oxide additiveselected from (a) a metal wherein said metal is selected from Ti, Mn,Sn, Pb, Ru, Co, Fe, Cu and Cr; (b) a metal oxide, MOx, wherein M isselected from Ti, Mn, Sn, Pb, Ru, Co, Fe, Cu and Cr; (c) any compoundsthat can generate the metal oxides of (b) upon firing; and (d) mixturesthereof.

A. Electrically Functional Silver Powders

Generally, a thick film composition comprises a functional phase thatimparts appropriate electrically functional properties to thecomposition. The functional phase comprises electrically functionalpowders dispersed in an organic medium that acts as a carrier for thefunctional phase that forms the composition. The composition is fired toburn out the organic phase, activate the inorganic binder phase and toimpart the electrically functional properties.

The functional phase of the composition may be coated or uncoated silverparticles which are electrically conductive. When the silver particlesare coated, they are at least partially coated with a surfactant. Thesurfactant may be selected from, but is not limited to, stearic acid,palmitic acid, a salt of stearate, a salt of palmitate and mixturesthereof. Other surfactants may be utilized including lauric acid,palmitic acid, oleic acid, stearic acid, capric acid, myristic acid andlinolic acid. The counter-ion can be, but is not limited to, hydrogen,ammonium, sodium, potassium and mixtures thereof.

The particle size of the silver is not subject to any particularlimitation, although an average particle size of no more than 10microns, and preferably no more than 5 microns, is desirable. The silverpowder accounts for 70 to 85 wt % of the paste composition, andgenerally 92 to 99 wt % of the solids in the composition (i.e.,excluding the organic vehicle).

B. Zn-Containing Additive(s)

The Zn-containing additive of the present invention may be selected from(a) Zn, (b) metal oxides of Zn, (c) any compounds that can generatemetal oxides of Zn upon firing, and (d) mixtures thereof. In addition,the Zn-containing additive has an average particle size of less than 0.1μm. In particular the Zn-containing additive has an average particlesize in the range of 7 nanometers to less than 100 nanometers.

C. Glass Frit

Examples of the glass frit compositions which may be used in the presentinvention include amorphous, partially crystallizable lead silicateglass compositions as well as other compatible glass frit compositions.In a further embodiment these glass frits are Cadmium-free. Some glassfrit compositions useful in the present invention are detailed in Table1 below. TABLE 1 Glass Frit Compositions (Weight Percent Total GlassComposition) ID # SiO₂ Al₂O₃ PbO B₂O₃ CeO2 ZnO Na2O Bi2O3 Li₂O TiO₂ CaOCdO 1 28.00 4.70 55.90 8.10 3.30 2 26.06 6.69 50.96 8.94 2.79 4.56 323.37 0.41 59.75 7.93 2.31 0.04 6.20 4 22.97 1.54 60.62 8.40 2.46 4.02 523.00 0.40 58.80 7.80 6.10 3.90 6 1.50 14.90 0.10 1.0 81.5 1.0

The glass compositions in weight % are shown in Table 1. Preferredcadmium-free glass compositions found in the examples comprise thefollowing oxide constituents in the compositional range of: SiO₂ 21-29,Al₂O₃ 0.1-8, PbO 50-62, B₂O₃ 7-10, ZnO 0-4, Li₂O 0-0.1, TiO₂ 2-7 inweight %. The more preferred composition of glass being: SiO₂ 28.00,Al₂O₃ 4.70, PbO 55.90, B₂O₃ 8.1, TiO₂ 3.30 in weight %.

Additionally, in a further embodiment, the glass frit composition is alead-free composition. Some lead-free glass compositions useful in thepresent invention comprise the following oxide constituents in thecompositional range of: SiO₂ 0.1-8, Al₂O₃ 0-4, B₂O₃ 8-25, CaO 0-1, ZnO0-42, Na₂O 0-4, Li₂O 0-3.5, Bi₂O₃ 28-85, Ag₂O 0-3 CeO₂ 0-4.5, SnO₂0-3.5, BiF₃ 0-15 in weight percent total glass composition.

An average particle size of the glass frit of the present invention isin the range of 0.5-1.5 μm in practical applications, while an averageparticle size in the range of 0.8-1.2 μm is preferred. The softeningpoint of the glass frit (Tc: second transition point of DTA) should bein the range of 300-600° C. The amount of glass frit in the totalcomposition is less than 4 wt. % of the total composition.

The glasses described herein are produced by conventional glass makingtechniques. More particularly, the glasses may be prepared as follows.Glasses are typically prepared in 500-1000 gram quantities. Typically,the ingredients are weighted, then mixed in the desired proportions, andheated in a bottom-loading furnace to form a melt in a platinum alloycrucible. Heating is typically conducted to a peak temperature(1000-1200° C.) and for a time such that the melt becomes entirelyliquid and homogeneous. The glass melts are then quenched by pouring onthe surface of counter rotating stainless steel rollers to form a 10-20mil thick platelet of glass or by pouring into a water tank. Theresulting glass platelet or water quenched frit is milled to form apowder with its 50% volume distribution set between 1-5 microns. Theresulting glass powders are formulated with filler and medium into thickfilm pastes or castable dielectric compositions.

D. Additional Metal/Metal Oxide Additives

The additional metal/metal oxide additives of the present invention maybe selected from (a) a metal wherein said metal is selected from Ti, Mn,Sn, Pb, Ru, Co, Fe, Cu and Cr, (b) a metal oxide MOx, wherein M isselected from Ti, Mn, Sn, Pb, Ru, Co, Fe, Cu and Cr, (c) any compoundsthat can generate the metal oxides of (b) upon firing, and (d) mixturesthereof.

The particle size of the additional metal/metal oxide additives is notsubject to any particular limitation, although an average particle sizeof no more than 10 microns, and preferably no more than 5 microns, isdesirable.

In one embodiment, the particle size of the metal/metal oxide additiveis in the range of 7 nanometers (nm) to 125 nm. In particular, MnO₂ andTiO₂ may be utilized in the present invention with an average particlesize range (d₅₀) of 7 nanometers (nm) to 125 nm.

The range of the metal/metal oxide additives and Zn-containing additivein the composition is 0.1 wt. % to 6 wt. % in the total composition.

Oxides such as MnOx and Cu/CuOx, as well as other oxides, may also aidin adhesion to some degree.

E. Organic Medium

The inorganic components are typically mixed with an organic medium bymechanical mixing to form viscous compositions called “pastes”, havingsuitable consistency and rheology for printing. A wide variety of inertviscous materials can be used as organic medium. The organic medium mustbe one in which the inorganic components are dispersible with anadequate degree of stability. The rheological properties of the mediummust be such that they lend good application properties to thecomposition, including: stable dispersion of solids, appropriateviscosity and thixotropy for screen printing, appropriate wettability ofthe substrate and the paste solids, a good drying rate, and good firingproperties. The organic vehicle used in the thick film composition ofthe present invention is preferably a nonaqueous inert liquid. Use canbe made of any of various organic vehicles, which may or may not containthickeners, stabilizers and/or other common additives. The organicmedium is typically a solution of polymer(s) in solvent(s).Additionally, a small amount of additives, such as surfactants, may be apart of the organic medium. The most frequently used polymer for thispurpose is ethyl cellulose. Other examples of polymers includeethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose andphenolic resins, polymethacrylates of lower alcohols, and monobutylether of ethylene glycol monoacetate can also be used. The most widelyused solvents found in thick film compositions are ester alcohols andterpenes such as alpha- or beta-terpineol or mixtures thereof with othersolvents such as kerosene, dibutylphthalate, butyl carbitol, butylcarbitol acetate, hexylene glycol and high boiling alcohols and alcoholesters. In addition, volatile liquids for promoting rapid hardeningafter application on the substrate can be included in the vehicle.Various combinations of these and other solvents are formulated toobtain the viscosity and volatility requirements desired.

The polymer present in the organic medium is in the range of 8 wt. % to11 wt. % of the total composition. The thick film silver composition ofthe present invention may be adjusted to a predetermined,screen-printable viscosity with the organic medium.

The ratio of organic medium in the thick film composition to theinorganic components in the dispersion is dependent on the method ofapplying the paste and the kind of organic medium used, and it can vary.Usually, the dispersion will contain 70-95 wt % of inorganic componentsand 5-30 wt % of organic medium (vehicle) in order to obtain goodwetting.

Description of Method of Manufacturing a Semiconductor Device

Accordingly, the invention provides a novel composition(s) that may beutilized in the manufacture of a semiconductor device. The semiconductordevice may be manufactured by the following method from a structuralelement composed of a junction-bearing semiconductor substrate and asilicon nitride insulating film formed on a main surface thereof. Themethod of manufacture of a semiconductor device includes the steps ofapplying (typically, coating and printing) onto the insulating film, ina predetermined shape and at a predetermined position, the conductivethick film composition of the present invention having the ability topenetrate the insulating film, then firing so that the conductive thickfilm composition melts and passes through the insulating film, effectingelectrical contact with the silicon substrate. The electricallyconductive thick film composition is a thick-film paste composition, asdescribed herein, which is made of a silver powder, Zn-containingadditive, a glass or glass powder mixture having a softening point of300 to 600° C., dispersed in an organic vehicle and optionally,additional metal/metal oxide additive(s).

The composition has a glass powder content of less than 5% by weight ofthe total composition and a Zn-containing additive combined withoptional additional metal/metal oxide additive content of no more than6% by weight of the total composition. The invention also provides asemiconductor device manufactured from the same method.

The invention may also be characterized by the use of a silicon nitridefilm or silicon oxide film as the insulating film. The silicon nitridefilm is typically formed by a plasma chemical vapor deposition (CVD) orthermal CVD process. The silicon oxide film is typically formed bythermal oxidation, thermal CFD or plasma CFD.

The method of manufacture of the semiconductor device may also becharacterized by manufacturing a semiconductor device from a structuralelement composed of a junction-bearing semiconductor substrate and aninsulating film formed on one main surface thereof, wherein theinsulating layer is selected from a titanium oxide silicon nitride,SiNx:H, silicon oxide, and silicon oxide/titanium oxide film, whichmethod includes the steps of forming on the insulating film, in apredetermined shape and at a predetermined position, a metal pastematerial having the ability to react and penetrate the insulating film,forming electrical contact with the silicon substrate. The titaniumoxide film is typically formed by coating a titanium-containing organicliquid material onto the semiconductor substrate and firing, or by athermal CVD. The silicon nitride film is typically formed by PECVD(plasma enhanced chemical vapor deposition). The invention also providesa semiconductor device manufactured from this same method.

The electrode formed from the conductive thick film composition(s) ofthe present invention is typically fired in an atmosphere that ispreferably composed of a mixed gas of oxygen and nitrogen. This firingprocess removes the organic medium and sinters the glass frit with theAg powder in the conductive thick film composition. The semiconductorsubstrate is typically single-crystal or multicrystalline silicon.

FIG. 1(a) shows a step in which a substrate of single-crystal silicon orof multicrystalline silicon is provided typically, with a texturedsurface which reduces light reflection. In the case of solar cells,substrates are often used as sliced from ingots which have been formedfrom pulling or casting processes. Substrate surface damage caused bytools such as a wire saw used for slicing and contamination from thewafer slicing step are typically removed by etching away about 10 to 20μm of the substrate surface using an aqueous alkali solution such asaqueous potassium hydroxide or aqueous sodium hydroxide, or using amixture of hydrofluoric acid and nitric acid. In addition, a step inwhich the substrate is washed typically with a mixture of hydrochloricacid and hydrogen peroxide may be added to remove heavy metals such asiron adhering to the substrate surface. An antireflective texturedsurface is sometimes formed thereafter using, for example, an aqueousalkali solution such as aqueous potassium hydroxide or aqueous sodiumhydroxide. This gives the substrate, 10.

Next, referring to FIG. 1(b), when the substrate used is a p-typesubstrate, an n-type layer is formed to create a p-n junction. Themethod used to form such an n-type layer may be phosphorus (P) diffusionusing phosphorus oxychloride (POCl₃). The depth of the diffusion layerin this case can be varied by controlling the diffusion temperature andtime, and is generally formed within a thickness range of about 0.3 to0.5 μm. The n-type layer formed in this way is represented in thediagram by reference numeral 20. Next, p-n separation on the front andbacksides may be carried out by the method described in the backgroundof the invention. These steps are not always necessary when aphosphorus-containing liquid coating material such as phosphosilicateglass (PSG) is applied onto only one surface of the substrate by aprocess, such as spin coating, and diffusion is effected by annealingunder suitable conditions. Of course, where there is a risk of an n-typelayer forming on the backside of the substrate as well, the degree ofcompleteness can be increased by employing the steps detailed in thebackground of the invention.

Next, in FIG. 1(d), a silicon nitride film or other insulating filmsincluding SiNx:H (i.e., the insulating film comprises hydrogen forpassivation) film, titanium oxide film, and silicon oxide film, 30,which functions as an antireflection coating is formed on theabove-described n-type diffusion layer, 20. This silicon nitride film,30, lowers the surface reflectance of the solar cell to incident light,making it possible to greatly increase the electrical current generated.The thickness of the silicon nitride film, 30, depends on its refractiveindex, although a thickness of about 700 to 900 Å is suitable for arefractive index of about 1.9 to 2.0. This silicon nitride film may beformed by a process such as low-pressure CVD, plasma CVD, or thermalCVD. When thermal CVD is used, the starting materials are oftendichlorosilane (SiCl₂H₂) and ammonia (NH₃) gas, and film formation iscarried out at a temperature of at least 700° C. When plasma CVD isused, pyrolysis of the starting gases at the high temperature results inthe presence of substantially no hydrogen in the silicon nitride film,giving a compositional ratio between the silicon and the nitrogen ofSi₃N₄ which is substantially stoichiometric. The refractive index fallswithin a range of substantially 1.96 to 1.98. Hence, this type ofsilicon nitride film is a very dense film whose characteristics, such asthickness and refractive index, remain unchanged even when subjected toheat treatment in a later step. The starting gas used when filmformation is carried out by plasma CVD is generally a gas mixture ofSiH₄ and NH₃. The starting gas is decomposed by the plasma, and filmformation is carried out at a temperature of 300 to 550° C. Because filmformation by such a plasma CVD process is carried out at a lowertemperature than thermal CVD, the hydrogen in the starting gas ispresent as well in the resulting silicon nitride film. Also, because gasdecomposition is effected by a plasma, another distinctive feature ofthis process is the ability to greatly vary the compositional ratiobetween the silicon and nitrogen. Specifically, by varying suchconditions as the flow rate ratio of the starting gases and the pressureand temperature during film formation, silicon nitride films can beformed at varying compositional ratios between silicon, nitrogen andhydrogen, and within a refractive index range of 1.8 to 2.5. When a filmhaving such properties is heat-treated in a subsequent step, therefractive index may change before and after film formation due to sucheffects as hydrogen elimination in the electrode firing step. In suchcases, the silicon nitride film required in a solar cell can be obtainedby selecting the film-forming conditions after first taking into accountthe changes in film qualities that will occur as a result of heattreatment in the subsequent step.

In FIG. 1(d), a titanium oxide film may be formed on the n-typediffusion layer, 20, instead of the silicon nitride film, 30,functioning as an antireflection coating. The titanium oxide film isformed by coating a titanium-containing organic liquid material onto then-type diffusion layer, 20, and firing, or by thermal CVD. It is alsopossible, in FIG. 1(d), to form a silicon oxide film on the n-typediffusion layer, 20, instead of the silicon nitride film 30 functioningas an antireflection layer. The silicon oxide film is formed by thermaloxidation, thermal CVD or plasma CVD.

Next, electrodes are formed by steps similar to those shown in FIGS.1(e) and (f). That is, as shown in FIG. 1(e), aluminum paste, 60, andback side silver paste, 70, are screen printed onto the backside of thesubstrate, 10, as shown in FIG. 1(e) and successively dried. Inaddition, a front electrode-forming silver paste is screen printed ontothe silicon nitride film, 30, in the same way as on the backside of thesubstrate, 10, following which drying and firing are carried out in aninfrared furnace at typically at a temperature range of 700 to 975° C.for a period of from several minutes to more than ten minutes whilepassing through the furnace a mixed gas stream of oxygen and nitrogen.

As shown in FIG. 1(f), during firing, aluminum diffuses as an impurityfrom the aluminum paste into the silicon substrate, 10, on the backside, thereby forming a p+ layer, 40, containing a high aluminum dopantconcentration. Firing converts the dried aluminum paste, 60, to analuminum back electrode, 61. The backside silver paste, 70, is fired atthe same time, becoming a silver back electrode, 71. During firing, theboundary between the backside aluminum and the backside silver assumesthe state of an alloy, thereby achieving electrical connection. Mostareas of the back electrode are occupied by the aluminum electrode,partly on account of the need to form a p+ layer, 40. At the same time,because soldering to an aluminum electrode is impossible, the silver orsilver/aluminum back electrode is formed on limited areas of the backside as an electrode for interconnecting solar cells by means of copperribbon or the like.

On the front side, the front electrode silver paste, 500, of theinvention is composed of silver, Zn-containing additive, glassfrit,organic medium and optional metal oxides, and is capable of reacting andpenetrating through the silicon nitride film, 30, during firing toachieve electrical contact with the n-type layer, 20 (fire through).This fired-through state, i.e., the extent to which the front electrodesilver paste melts and passes through the silicon nitride film, 30,depends on the quality and thickness of the silicon nitride film, 30,the composition of the front electrode silver paste, and on the firingconditions. The conversion efficiency and moisture resistancereliability of the solar cell clearly depend, to a large degree, on thisfired-through state.

EXAMPLES

The thick film composition(s) of the present invention are describedherein below in Examples 1-25.

Glass Preparation:

The glass composition(s) utilized in the Examples are detailed below inTable 2 and identified in Table 3. TABLE 2 Glass Compositions in WeightPercent of Total Glass Composition ASTM TMA Glass Composition in WeightPercent Softening Onset SiO2 Na2O Li2O Bi2O3 CeO2 Al2O3 PbO B2O3 TiO2ZnO Pt. (° C.) (° C.) Glass I 28.00 4.70 55.90 8.10 3.30 600 502 GlassII 6.00 80.50 12.00 1.50 430 365 Glass III 1.64 1.73 85.76 10.86 362 322Glass IV 9.10 1.40 77.0 12.50 395 361 Glass V 1.77 82.32 8.73 1.18 6.00— 340 Glass VI 1.50 1.0 1.0 81.50 0.10 14.90 — —Paste Preparation:

Paste preparations were, in general, accomplished with the followingprocedure: The appropriate amount of solvent, medium and surfactant wasweighed then mixed in a mixing can for 15 minutes, then glass frits andmetal additives were added and mixed for another 15 minutes. Since Ag isthe major part of the solids of the present invention, It was addedincrementally to ensure better wetting. When well mixed, the paste waspast through a 3-roll mill for a few times, under pressures from 0 to400 psi. The gap of the rolls were adjusted to 1 mil. The degree ofdispersion was measured by fineness of grind (FOG). A typical FOG valueis generally equal to or less than 20/15 for conductors. TABLE 3Examples 1-25 Silver and Glass Compositions in Weight Percent of TotalComposition Example Ag Glass I Glass II Glass III Glass IV Glass Number(wt %) (wt %) (wt %) (wt %) (wt %) VI 1 80 2.0 0 0 0 0 2 80.95 2.0 0 0 00 3 80 0.5 0 0 0 4 80.95 1.5 0.5 0 0 0 5 79.5 2.0 0.5 0 0 0 6 80 2.0 0 00 0 7 80 2.0 0 0 0 0 8 80 2.0 0 0 0 0 9 80.45 0 2.5 0 0 0 10 80.45 0 2.50 0 0 11 80.35 0 2.5 0 0 0 12 80.75 1.5 0.5 0 0 0 13 79.5 0 2.5 0 0 0 1479.0 2.0 0 0 0 0 15 79.45 0 2.5 0 0 0 16 80.35 0 0 2.5 0 0 17 81.95 0 00 0 0 18 84.45 0 0 0 2 0 19 80.0 0 2.0 0 0 20 80.45 2.0 0.5 0 0 0 21 790 0 0 0 1.4 22 80 2.0 0 0 0 0 23 80 2.0 0 0 0 0 24 80 2.0 0 0 0 0 25 802.0 0 0 0 0* Each of the Examples 1-25 contain 12 wt. % solvent (Texanol ®), 1.1wt. % ethyl cellulose and 0.8 wt. % surfactant (soya lecithin).

TABLE 4 Examples 1-20 Metal, Metal Resinate Compositions in WeightPercent of Total Composition Example ZnO Zn TiO2 Mn Zn Number ZnO finemetal TiO2 fine MnO2 Tyzor Resinate Resinate Cu Eff % Adhesion 1 4.45 00 0 0 0 0 0 0 0 15.3 good 2 0 3.50 0 0 0 0 0 0 0 0 15.6 good 3 4.45 0 00 0 0 0 0 0 0 15.4 good 4 0 3.50 0 0 0 0 0 0 0 0 15.4 good 5 4.45 0 0 00 0 0 0 0 0 15.4 good 6 4.45 0 0 1.0 0 0 0 0 0 0 15.3 adequate 7 4.45 00 0 0 0 1.0 0 0 0 15.4 good 8 4.45 0 0 0 0 0 0 1.0 0 0 15.1 good 9 03.50 0 0 0 0 0 0 0 0 15.4 v good 10 0 3.50 0 0 0.1 0 0 0 0 0 >15.6 good11 0 3.50 0 0 0 0 0 0 0 0.2 15.6 excellent 12 4.45 0 0 0 0 0 0 0 20 >15.3 good 13 4.45 0 0 0 0 1.0 0 0 0 0 >15 good 14 0 0 0 0 0 4.50 0 00 0 >15 good 15 0 3.5 0 0 0.1 0 0 0 0 0 ˜15 v good 16 0 3.5 0 0 0 0 0 00 1.0 >10 adequate 17 0 0 0 0 0 0 0 0 0 0 ˜4 not tested 18 4.45 0 0 0 00 0 0 0 0 ˜15 good 19 0 4.45 0 0 0 0 0 0 0 14.7 adequate 20 0 3.5 0 00.1 0 0 0 0 0.2 15.7 V good 21 6.5 0 0 0 0 0 0 0 0 0 ˜15 V goodEach of the Examples 1-20 contain 12 wt. % solvent (Texanol ®), 1.1 wt.% ethyl cellulose and 0.8 wt. % surfactant soya lecithin.** The ZnO fine is commercially available from US Zinc Corporation withaverage particle sizes of 7 nm and 30 nm.*** The Zn resinate is commercially available from OMG Americas, as ZINCTEN-CEM.

TABLE 5 Examples 22-25, Metal Oxide Compositions in Weight Percent ofTotal Composition, and Their Solar Cell Properties Example TiO2 Co3O4Fe2O3 Cr2O3 Eff % Adhesion 22 4.45 <5 NT 23 4.45 <5 NT 24 4.45 <5 NT 254.50 <5 NTNT: Not testedTest Procedure-Efficiency

The solar cells built according to the method described above wereplaced in a commercial IV tester for measuring efficiencies. The lightbulb in the IV tester simulated the sunlight with a known intensity andradiated the front surface of the cell, the bus bars printed in thefront of the cell were connected to the multiple probes of the IV testerand the electrical signals were transmitted through the probes to thecomputer for calculating efficiencies.

Test Procedure-Adhesion

After firing, a solder ribbon (copper coated with 62 Sn/36 Pb/2 Ag) wassoldered to the bus bars printed on the front of the cell. Soldercondition was typically at 345° C. for 5 seconds. Flux used was mildlyactivated alpha-611 or not activated multicore 100. The soldered areawas approximately 2 mm×2 mm. The adhesion strength was obtained bypulling the ribbon at an angle of 90° to the surface of the cell. Anassessment of the adhesion strength was assigned as adequate, good, verygood, or excellent, based on the assumption that an adhesion strengthless than 400 g is considered not good; values in the range of 400 g toless than 600 g were assessed as adequate adhesion strength; equal to orgreater than 600 g is considered good, very good, or excellent adhesionstrength.

For the compositions using Pb free frit, we tested adhesion using bothPb free solder and Pb containing solder. Pb free solder used is 96.5Sn/3.5 Ag. Solder temperature for the Pb free solder is 375° C., soldertime is 5-7 s. Flux used is MF200. Adhesion obtained is typically above600 g.

Additionally, after pulling, the solder joint was examined under amicroscope for failure mode. If more than 50% of the soldered areashowed Si fracture, it was considered a very good failure mode.

1. A thick film conductive composition comprising: a) electricallyconductive silver powder; b) zinc-containing additive wherein theparticle size of said zinc-containing additive is in the range of 7nanometers to less than 100 nanometers; c) glass frit wherein said glassfrit has a softening point in the range of 300 to 600° C.; dispersed ind) organic medium.
 2. The composition of claim 1 further comprising anadditional metal/metal oxide additive selected from (a) a metal whereinsaid metal is selected from Ti, Mn, Sn, Pb, Ru, Co, Fe, Cu and Cr; (b) ametal oxide, MOx, wherein M is selected from Ti, Mn, Sn, Pb, Ru, Co, Fe,Cu and Cr; (c) any compounds that can generate the metal oxides of (b)upon firing; and (d) mixtures thereof.
 3. The composition of claim 1wherein said Zn-containing additive is ZnO.
 4. The composition of claim1 wherein said glass frit composition comprises, based on weight percenttotal glass frit composition: SiO₂ 21-29, Al₂O₃ 0.1-8, PbO 50-62, B₂O₃7-10, ZnO 0-4, Li₂O 0-0.1, and TiO₂ 2-7.
 5. The composition of claim 1wherein said glass frit is a lead-free glass frit comprising, based onweight percent total glass frit composition, SiO₂ 0.1-8, Al₂O₃ 0-4, B₂O₃8-25, CaO 0-1, ZnO 0-42, Na₂O 0-4, Li₂O 0-3.5, Bi₂O₃ 28-85, Ag₂O 0-3CeO₂ 0-4.5, SnO₂ 0-3.5, and BiF₃ 0-15.
 6. A substrate wherein thecomposition of claim 1 has been deposited and wherein said compositionhas been processed to remove said organic medium and sinter said glassfrit and silver powder.
 7. An electrode formed from the composition ofclaim 1 wherein said composition has been processed to remove saidorganic medium and sinter said glass frit and silver powder.
 8. A methodof manufacturing a semiconductor device from a structural elementcomposed of a semiconductor having a p-n junction and an insulating filmformed on a main surface of the semiconductor comprising the steps of(a) applying onto said insulating film the thick film composition ofclaim 1; and (b) firing said semiconductor, insulating film and thickfilm composition to form an electrode.
 9. The method of claim 8, whereinthe insulating film is selected from the group comprising siliconnitride film, titanium oxide film, SiNx:H film, silicon oxide film and asilicon oxide/titanium oxide film.
 10. A semiconductor device formed bythe method of claim
 8. 11. A semiconductor device comprising from thecomposition of claim 1 wherein said composition has been processed toremove said organic medium and sinter said glass frit and silver powder.